2.1. IMMUNE RESPONSE IN VIRAL INFECTIONS
After a virus invades a host cell, it begins to replicate. With DNA viruses, specific virus DNA strands are transcribed into specific messenger RNA (mRNA) which is then translated to synthesize virus-specific proteins and enzymes necessary for biosynthesis of virus DNA. With RNA viruses, single-stranded RNA serves as its own messenger. The mRNA is translated, resulting in the formation of an RNA polymerase. Some of the viral proteins are structural parts of the capsid and others are virus-specific enzymes.
The host responds to the viral invasion in a cascade type of response. Macrophages and polymorphonuclear leukocytes migrate to the area of infection, producing inflammation. Interferons are produced locally, and B, T, and null lymphocytes appear. Macrophages attach to antigen, markedly augmenting the efficacy of virus inactivation by specific neutralizing antibodies or sensitized T lymphocytes. During the first few days of many virus infections there is a circulating lymphopenia of both B and T cells. Lymphocytes migrate to sites of infection and to the liver, spleen, or other organs of the mononuclear phagocytic system.
Proteins of the virus capsid stimulate B lymphocytes (immunoglobulin-bearing, bone marrow-derived) to synthesize humoral antibodies (IgM, IgG, IgA, IgD) and secretory antibodies (IgA). Cell-associated antibodies (IgE) are also produced. Immunoglobulins are synthesized in local lymph nodes, at body surfaces, and in inflammatory exudates within organs. Many molecules of antibody combine with a single virion. Covering a critical number of essential sites on the virion may render an antigen-antibody complex noninfectious.
There are few drugs that exist which effectively combat viruses per se, owing to the difficulty of attacking the virus while leaving uninfected host cells unimpaired. Prevention of some virus infections has instead been achieved primarily through the development of viral vaccines, either live attenuated, and "killed" or inactivated viruses. Passive immunization with pooled human immune serum globulins or globulins with high titers of antibodies to specific agents such as rabies or hepatitis B have also been of some benefit in the prophylaxis of viral infections. These preparations generally are most useful when given early during the incubation period in order to prevent clinical illness.